Conventionally, a polishing apparatus of this type includes a polishing-tape supply reel mechanism, a polishing head, a polishing-tape recovery reel mechanism, and a polishing-tape supply and recovery mechanism for recovering a polishing tape supplied from the polishing-tape supply reel mechanism via the polishing head to the polishing-tape recovery reel mechanism. The polishing tape is brought into contact with a periphery of a workpiece, such as a semiconductor wafer, while traveling through the polishing head. The workpiece is polished by relative movement between the polishing tape and the workpiece.
FIG. 1 is a schematic view showing the above-mentioned polishing apparatus, and FIG. 2 is a plan view showing the polishing-tape supply and recovery mechanism of the polishing apparatus. As shown in the drawings, a polishing apparatus 100 includes a polishing-tape supply reel mechanism 101, a polishing head 103, and a polishing-tape recovery reel mechanism 102. A polishing tape 105 is supplied from the polishing-tape supply reel mechanism 101 via a guide roller 104 to the polishing head 103. The polishing tape 105 travels through the polishing head 103 to a guide roller 106, and is wound and recovered by the polishing-tape recovery reel mechanism 102. A periphery of a substrate (e.g., a semiconductor wafer) W, held by a substrate holding stage 120, is brought into contact with the polishing tape 105 traveling through the polishing head 103, and the periphery of the substrate W is polished by relative movement between the polishing tape and the workpiece.
The polishing-tape supply reel mechanism 101 and the polishing-tape recovery reel mechanism 102 are driven by a drive motor 107 and a drive motor 108, respectively. The drive motor 107 and the drive motor 108 are provided with a rotary encoder 109 and a rotary encoder 110 for detecting a rotation angle of the drive motor 107 and the drive motor 108, respectively. A rotational torque of the drive motor 107 and a rotational torque of the drive motor 108 are controlled so as to maintain a constant tension exerted on the polishing tape 105. The polishing tape 105 is contained as a polishing-tape roll 111 between a reel plates 114a and 114b of the polishing-tape supply reel mechanism 101. The recovered polishing tape 105 is contained as a polishing-tape roll 111 between a reel plates 115a and 115b of the polishing-tape recovery reel mechanism 102.
As shown in FIG. 3, the polishing tape 105 is wound as the polishing-tape roll 111 on a cylindrical core 101a of the polishing-tape supply reel mechanism 101. This core 101a has an inside diameter Dci and an outside diameter Dco. Since the new polishing-tape supply reel mechanism 101 supplies the polishing tape 105, the outside diameter of the roll 111 is gradually decreased. On the other hand, since the polishing-tape recovery reel mechanism 102 winds the polishing tape 105, the outside diameter of the roll 111 is gradually increased. If the drive motors 107 and 108 keep their rotational torque constant, the tension exerted on the polishing tape 105 changes, as the outside diameter of the polishing-tape roll 111 changes as the result of consumption of the polishing tape 105. The tension of the polishing tape 105 acts as a polishing load between the polishing tape 105 and the substrate W, i.e., a workpiece to be polished. Therefore, in order to keep the polishing load constant irrespective of the consumption of the polishing tape 105, it is necessary to keep the tension of the polishing tape 105 constant regardless of a change in outside diameter of the roll 111 of the polishing tape 105. Thus, it is necessary to control an output of the drive motor 107 and an output of the drive motor 108 in accordance with the change in outside diameter of the roll 111 of the polishing tape 105, so as to control the rotational torque to be exerted on the polishing-tape supply reel mechanism 101 and the polishing-tape recovery reel mechanism 102.
FIG. 4 shows a conventional mechanism of detecting the outside diameter of the roll 111 of the polishing tape 105 on the polishing-tape supply reel mechanism 101 and the polishing-tape recovery reel mechanism 102. As shown in FIG. 4, an outside-diameter sensor 112 and an outside-diameter sensor 113, as a pair of laser sensors, are provided so as to interpose the polishing-tape roll 111 on the polishing-tape supply reel mechanism 101 therebetween. The outside-diameter sensor 112 is a light-emitting sensor, and the outside-diameter sensor 113 is a light-receiving sensor. A distance of light interruption by the roll 111 of the polishing tape 105 is detected and the distance is converted into the outside diameter of the polishing tape 105. In the example shown in FIG. 4, the outside-diameter sensors 112 and 113 are provided so as to measure only the outside diameter of the roll 111 of the polishing tape 105 on the polishing-tape supply reel mechanism 101.
In this method of detecting the outside diameter of the roll 111 of the polishing tape 105, it is necessary to provide the outside-diameter sensors for both the polishing-tape supply reel mechanism 101 and the polishing-tape recovery reel mechanism 102. When using an optical sensor like the conventional technique, it is necessary to adjust, for each of the reels, a relational expression for converting the amount of light interruption, from the light emitting to the light receiving, into the outside diameter of the polishing-tape roll 111. In addition, since the outside-diameter sensors 112 and 113 are provided near the polishing-tape supply reel mechanism 101 and the polishing-tape recovery reel mechanism 102, the outside-diameter sensors 112 and 113 could hinder replacement operations of the polishing tape 105.